Scientists from Sony Corporation in Japan have recently developed biofuel cells or bio-batteries running on sugar that can power small gadgets like MP3 players. Prior to this, similar technologies haven't been used for practical applications because the energy produced is too little. For this particular project, the researchers used enzymes to increase the output.

Biofuel cells consist of an anode, a cathode, and a proton-conducting membrane in between. Glucose as a fuel, for example, undergoes oxidation at the anode part to separate the protons and electrons. While the protons go through the membrane to reach the cathode, the electrons need to travel via an external circuit towards the cathode so the two components could rejoin and combine with oxygen to form water. Unfortunately, the electrons' travel is quite slow, so an electron transfer mediator, particularly naphthoquinone derivative, is used to shuttle the electrons. This mediator which is packed with enzymes increases the density and hence, the energy output.

Stacking four cells together is able to produce 100 milliwatts of energy, enough to power an ordinary MP3 player with speakers or even a remote-controlled car.